1. Field of the Invention
The present invention relates to a frequency-modulated reference frequency generator with overshoot compensation in switching between two frequencies about a predetermined reference frequency.
2. Description of the Prior Art
Phase-controlled oscillators (PLL-oscillators) can be frequency-modulated either by corresponding action on the phase-locked loop or by corresponding frequency modulation of the reference frequency applied to the phase detector. In the latter case of frequency modulation more or less intense distortions of the data signals may occur due to overshooting and transients during the frequency change. This occurs to a significant degree in PLL-synthesizers in which the parameters for the phase-locked loop must be dimensioned for optimum fast locking such as is required, for instance, for controlling communication transmitters operating on the principle of frequency hopping. Moreover, because of the interfering overshooting the modulation spectrum is unnecessarily broadened, and a strong hangover may occur in the intermediate-frequency filter of a receiver which receives the modulated signal, whereby also the detectability is deteriorated and consequently the bit error ratio is increased. To avoid this drawback it has been necessary with known synthesizers of this kind either to change the loop parameters or to reduce the data rate, which means that previously such distortions could be prevented only at the cost of a deterioration in the synthesizer performance.
Purely digital synthesizers are also known in which the output signal is generated by direct digital synthesis (Neues von Rohde & Schwarz, No. 123, fall of 1988, p. 16, U.S. Pat. No. 3,735,269, fully incorporated herein by reference DE-PS 3,740,130). Such digital synthesizers may also be frequency-modulated in a simple way by a digital data signal (square-wave pulses), and to this end it is only required to allocate to the control input of the synthesizer a corresponding register in which the two drift frequencies, which are adjacent a centre frequency and determine the frequency deviation, are stored as numerical values which are alternately read out for modulation in response to the data signal. Such purely digital synthesizers, however, have the drawback of a low degree of freedom from spurious frequencies.